It is not recommended to perturb the
beer between when fermentation starts
and kegging/bottling because you run a
strong risk of coming into too much
contact with oxygen. Carefully racking
to secondary with minimal oxygen
exposure will leave you with plenty of
yeast for carbonation.

Please see Why do you aerate wort at
first and try to keep oxygen out
later? for why oxygen is not good for
wort/beer once fermentation has begun.

5 Answers
5

Watch your temperatures! - I accidentally burned a batch a bit by just having the stove cranked up for most of the time. Really when you place in your barley you want to steep, not boil, like tea. This can easily burn and get some odd tastes. Malts will immediately boil over if the temp is too high. Hops will dissipate faster if kept on high temps. When a recipe calls for boil get it just to a gentle boil, not the crazy exploding boil, more flavors will be released and maintained.

Yeast is your friend - Yeast is actually very resilient and just wants to eat. Just be sure to not let it get shocked hot or cold.

Taste - You can taste your beer before it's in the bottle! Throughout the whole process just take a little bit and give it a good taste. While some things will change throughout the process in general what you're tasting is what you're gonna get.

Don't panic! - Even if you think you've screwed something up keep going. It'll still be beer, hell it may be an awesome beer that you didn't know you were making.

If I had to pick the one critical variable that's easiest to screw up, and has the biggest effect on the final beer, I would say it's fermentation temperature. Fermenting too hot, too cold, or quickly changing the temperature during the first 72 hours of fermentation will have drastic effects on your beer. While it does seem to be the easiest thing to blow, luckily, it's also fairly easy to control, if you're careful and resourceful.

Next, I'd say sanitation is key. It's cliche, but true.

Other important aspects are hitting your target gravity (which is more complicated in all-grain), pitching the right volume of healthy yeast, adequate aeration, and proper carbonation (either with priming sugar or proper regulator settings).

99% of the time secondary is completely unnecessary. It carries a very small risk of harming the beer, so why bother? And I completely agree that fermentation temp and sanitation are about the 2 most critical aspects.

I think the most critical step is making sure you pitch a enough healthy yeast for the gravity of beer you brewing. If you have a large population of healthy yeast in your wort that can make up for other process shortfalls like under aeration, low fermentation temperature, and even less than perfect sanitation.

This is not to say you should not focus on the other steps like fermentation control, wort aeration, sanitation, and wort chilling but the yeast is the workhorse. The yeast affect the outcome of a beer more than any other ingredient or process.

Document everything you do when brewing — This is the first step of the way towards being able to successfully reproduce a brew. Take measurements during the entire process, and write down all weights, volumes, temperatures and gravities.

Follow a recipe — Know before you start brewing how much you will use of each ingredient, mash temperature and time, boil time, desired OG etc. Document any deviations.

Get to know your equipment — By documenting rigorously, after a few brews you will have enough data to know how much water to use, how to best keep a desired mash temperature, your mash efficiency, evaporation rate etc. Then you will be able to achieve your desired result every time.

If you do all this, then congratulations, you are a skilled home brewer and you will be able to make the same beer twice.